Constant velocity joints are used in front engine/front wheel drive cars, in cars with independent suspension, or in 4-wheel drive vehicles. The constant velocity joints (CVJs) are special types of universal couplings which transmit drive from the final reduction gear to a road wheel axle at constant rotational velocity. The two major categories of constant velocity joint are plunging and fixed constant velocity joints and are usually used in a vehicle in suitable combinations. The plunging CVJs allow sliding in the axial direction, while fixed CVJs do not permit movement in the axial direction. The mechanical components of plunging joints undergo complex rolling and sliding motions when the joint is at an angle and undergoing rotation and it is known that the frictional resistance to these motions can cause the motor vehicle to suffer vibrations, acoustic beating noises, and small rolling motions, particularly under certain driving conditions. Such noise, vibrations, and motions can be unpleasant to the vehicle occupants.
Accordingly, attempts have been made to formulate CVJ greases to improve their frictional characteristics so as to reduce the frictional forces within plunging constant velocity joints and noise and vibrations experienced in cars. A number of studies have shown there to be useful correlations between these noises and vibrations and the friction coefficients measured in certain laboratory friction testers. In particular, the SRV (Schwingungs Reibung und Verschleiss) laboratory friction tester (manufactured by Optimol Instruments) has been found in a number of studies to provide a useful guide in the development of low friction constant velocity joint greases for improved noise and vibration.
Examples of lubricating greases commonly used in such constant velocity joints include a grease comprising a calcium complex soap as a thickening agent; a grease comprising a lithium soap as thickening agent; a grease comprising a lithium complex as thickening agent; and a grease comprising a polyurea as thickening agent. However, thickeners may also be one of a variety of materials, including clays, and fatty acid soaps of calcium, sodium, aluminium, and barium.
The base oils used in lubricating greases are essentially, the same type of oil as would normally be selected for oil lubrication. The base oils may be of mineral and/or synthetic origin. Base oils of mineral origin may be mineral oils, for example produced by solvent refining or hydroprocessing. Base oils of synthetic origin may typically be mixtures of C.sub.10-50 hydrocarbon polymers, for example liquid polymers of alpha-olefins. They may also be conventional esters for example polyol esters. The base oil may also be a mixture of these oils. Preferably the base oil is that of mineral origin sold by the Royal Dutch/Shell Group of Companies under the designations "HVI" or "MVIN", is a polyalphaolefin, or a mixture thereof. Base oils of the type manufactured by the hydroisomerisation of wax, such as those sold by the Royal Dutch/Shell Group of Companies under the designation "XHVI" (trade mark) may also be included.
The lubricating grease preferably contains 2 to 20% by weight of thickener, preferably 5 to 20% by weight.
Lithium soap thickened greases have been known for many years. Typically, the lithium soaps are derived from C.sub.10-24, preferably C.sub.15-18, saturated or unsaturated fatty acids or derivatives thereof. One particular derivative is hydrogenated castor oil, which is the glyceride of 12-hydroxystearic acid.
12-hydroxystearic acid is a particularly preferred fatty acid.
Greases thickened with complex thickeners are well known. In addition to a fatty acid salt, they incorporate into the thickener a complexing agent which is commonly a low to medium molecular weight acid or dibasic acid or one of its salts, such as benzoic acid or boric acid or a lithium borate.
Urea compounds used as thickeners in greases include the urea group (--NHCONH--) in their molecular structure. These compounds include mono-, di- or polyurea compounds, depending upon the number of urea linkages.
Various conventional grease additives may be incorporated into the lubricating greases, in amounts normally used in this field of application, to impart certain desirable characteristics to the grease, such as oxidation stability, tackiness, extreme pressure properties and corrosion inhibition. Suitable additives include one or more extreme pressure/antiwear agents, for example zinc salts such as zinc dialkyl or diaryl dithiophosphates, borates, substituted thiadiazoles, polymeric nitrogen/phosphorus compounds made, for example, by reacting a dialkoxy amine with a substituted organic phosphate, amine phosphates, sulphurised sperm oils of natural or synthetic origin, sulphurised lard, sulphurised esters, sulphurised fatty acid esters, and similar sulphurised materials, organo-phosphates for example according to the formula (OR).sub.3 P=O where R is an alkyl, aryl or aralkyl group, and triphenyl phosphorothionate; one or more overbased metal-containing detergents, such as calcium or magnesium alkyl salicylates or alkylarylsulphonates; one or more ashless dispersant additives, such as reaction products of polyisobutenyl succinic anhydride and an amine or ester; one or more antioxidants, such as hindered phenols or amines, for example phenyl alpha naphthylamine, diphenylamine or alkylated diphenylamine; one or more antirust additives such as oxygenated hydrocarbons which have optionally been neutralised with calcium, calcium salts of alkylated benzene sulphonates and alkylated benzene petroleum sulphonates, and succinic acid derivatives, or friction-modifying additives; one or more viscosity-index improving agents; one or more pour point depressing additives; and one or more tackiness agents. Solid materials such as graphite, finely divided MoS.sub.2, talc, metal powders, and various polymers such as polyethylene wax may also be added to impart special properties.
Studies with oil soluble molybdenum dithiocarba-mates (MoDTC's) (PCH Mitchell, Wear 100 (1984) 281; H Isoyama and T Sakurai, Tribology International 7 (1974) 151; E R Braithwaite and A B Greene, Wear 46 (1978) 405; and Y Yamamoto and S Gondo, Tribology Trans., 32 (1989) 251) and with other organomolybdenum compounds in the presence of sulphur containing materials (Y Yamamoto, S Gondo, T Kamakura and M Konishi, Wear 120 (1987) 51; Y Yamamoto, S Gondo, T Kamakura and N Tanaka, Wear 112 (1986) 79; A B Greene and T J Ridson SAE Technical Paper 811187 Warrendale Pa., 1981; and I Feng, W Perilstein and M R Adams ASLE Trans., 6 (1963) 60) have been shown to be effective in reducing friction and wear. The presence of molybdenum in combination with sulphur (A. B. Greene and T. J. Ridson SAE Technical Paper 811187 Warrendale Pa., 1981), and possibly phosphorous (Y Yamamoto, S Gondo, T Kamakura and M Konishi, Wear 120 (1987) 51), appear to be necessary conditions for the achievement of low friction. The source of sulphur may be from an additive used in combination with the molybdenum compound (K Kubo, Y Hamada, K Moriki and M Kibukawa, Japanese Journal of Tribology, 34 (1989) 307), commonly zinc dithiophosphate (ZnDTP), from the base oil used (Y Yamamoto, S Gondo, T Kamakura and N Tanaka, Wear 112 (1986) 79) or through chemical combination with the molybdenum compound itself (as is the case for MoDTC).
However there are many instances in the literature where the addition of organomolybdenum--sulphur compounds to oils produced no reduction in friction. The source of sulphur used in combination with the organomolybdenum appears to be critical; some ZnDTP types produce a fall in friction, while others cause a rise in friction (K Kubo, Y Hamada, K Moriki and M Kibukawa, Japanese Journal of Tribology, 34 (1989) 307).
In an NTN study (SAE Technical Paper 871985; The Development of Low Friction and Anti-Fretting Corrosion Greases for CVJ and Wheel Bearing Applications, M Kato and T Sato of NTN Toyo Co Ltd), the largest reduction in friction was found when molybdenum dithiophosphate (MoDTP) was included with ZnDTP in a polyurea base grease. The addition of MoDTC together with ZnDTP to polyurea grease brought about a smaller reduction in friction.
WO 97/03152 discloses a lubricating composition comprising a base oil, molybdenum disulphide, zinc naphthenate and zinc dithiophosphate, and optionally zinc dithiocarbamate. There is no information in this document from which can be derived that the combination of compounds according to the present invention, is a good friction reduction agent.